Not applicable.
1. Field of the Invention
This invention relates to pneumatic conveying and, in particular, to the conveying of materials which are in the form of thick, heavy pastes which are normally difficult to move and handle.
2. Description of the Related Art
An example of the sort of material with which this invention is concerned is provided by the oil exploration industry. When oil wells are drilled, the cuttings from the drilling operations are brought up onto the drilling platform. For a large part of the drilling operation, a special type of oil is pumped down to the drilling bits as a lubricant. The oil contaminated material which comes up onto the drilling platform has until recently been dumped into the sea. For environmental reasons, such disposal is no longer permitted and the material now has to be transported to the shore for processing.
On the drilling rig, the oil contaminated cuttings are screened to remove a high proportion of the oil for re-use on the rig. The cuttings, which are still contaminated with some oil, are transported ashore in the form of a very thick heavy paste. Typically the material is put into special skips of about 10 ton capacity which are loaded by crane from the rig onto supply boats. This is a difficult and dangerous operation in bad weather and is laborious and expensive.
German Patent Application No. 4010676 describes a device for even, continuous conveying of stiff, solids-containing compositions from a pressure vessel into a pipe- or hose-line wherein, at the outlet of the pressure vessel the material to be conveyed is mixed with the conveying air in a nozzle, while the material to be conveyed is fed evenly to the nozzle by means of a conveying screw. However, such a device suffers from the disadvantage that mechanical agitation is required.
We have now surprisingly found a novel method of transferring thick heavy pastes material e.g. drill cuttings which has not been previously possible.
Accordingly, the present invention is based on surprising discovery that it is possible to transport a material in the form of a thick heavy paste by means of pneumatic conveying from a relatively large vessel. Hitherto, it has been supposed that pneumatic conveying systems were only suitable for relatively free flowing material or conveying of small batches of wet sticky materials.
Thus, according to the present invention there is provided a method of conveying a non-free flowing paste comprising loading the paste into a transportable vessel and applying a compressed gas to the vessel to cause the material to flow out of the vessel.
The non-free flowing paste may be a thick and/or heavy paste or clay like material, e.g. oil rig drill cuttings.
Thus, the transportable vessel is preferably a combined storage and pneumatic conveying vessel.
The compressed gas is preferably compressed air, because it is relatively inexpensive although in certain instances an inert gas may be used, for example, compressed nitrogen.
The vessel is provided with an inlet and an outlet, such that loading of the vessel is via the inlet. Preferentially the outlet is connected to a conduit which leads to the final desired destination of the material.
In a preferred embodiment the method of the invention also includes the step of transporting the vessel, having at least partially filled it with said material, from its filling station to a discharge station. At the discharge station, compressed air is applied to the interior of the vessel to convey the material out of the vessel to its destination.
Preferably the vessel includes a conical hopper portion which, at least during discharge of the material, forms the lower section of the vessel. In a further preferred embodiment, the lower conical hopper portion is the outlet end of the vessel.
Because of the nature of the material being handled in the method of the present invention, there is a tendency for the flow of the material out of the vessel to be less than complete. This is because the type of flow which occurs during discharge is of a form known as core flow or funnel flow. When this type of flow occurs, the material directly above the outlet falls through the outlet, e.g. the outlet valve, so that a falling core of material is created directly above the outlet. However, with sticky materials, the material around this core does not move. As the core falls, a depression occurs in the top surface of the material and the material surrounding that depression falls into the core. In the case of a sticky material there is a tendency for material around the core to remain in the vessel.
It has been discovered that even for the sort of material with which this invention is concerned, it is possible to change the nature of the flow out of the vessel by altering the cone angle of the lower conical portion of the vessel. If the included cone angle is changed below a certain or critical value, then the flow changes from core flow to so-called mass flow. In the case of mass flow, the material descends as a mass in a uniform way towards the outlet with all the material moving. Accordingly the combination of the use of a vessel designed to achieve mass flow and the application of a compressed gas above the surface of the material is such that it is possible to push the contents of the vessel through the outlet so as fully to empty the vessel.
It is well known that the critical cone angle will vary depending upon the material being conveyed as such would be well understood by those skilled in the art.
The pressure used in the vessel in the method may also vary depending upon the nature of the material. However, we have found that a pressure of between 4 and 8 bar is suitable.
In an embodiment of the present invention the drill cuttings are loaded into a first relatively small vessel capable of being pressurised from which said material is fed under pressure via a pipe to one or more further vessels also capable of being pressurised. Said further vessels may be transported to a position where discharge of material takes place or said further vessels may remain in their original position and the material is discharged from them into yet further vessels which are themselves transported to the destination.
The present invention also provides apparatus for conveying a material in the form of a thick, heavy paste, the apparatus comprising a vessel capable of being pressurised by compressed gas, said vessel having a material inlet, a material outlet and a pipe connected to said material outlet, means for loading the material into said vessel through said inlet, gas supply means for supplying compressed gas to said vessel to cause the material to flow out of the vessel via said outlet and along said pipe.
Conventionally known pressure vessels are expected to withstand a maximum pressure of 2 bar. Thus, in a preferred embodiment the apparatus of the invention comprises a vessel adapted to be pressurised by compressed gas to between 4 and 8 bar.